1. Field of the Invention
The present invention relates to methods for making raw ceramic powders, the raw ceramic powders, dielectric ceramics produced using the raw ceramic powders, and monolithic ceramic electronic components using the dielectric ceramics.
2. Description of the Related Art
As monolithic ceramic capacitors have been miniaturized, the thickness of dielectric ceramic layers has been decreased to approximately 3 xcexcm. Use of base metals, such as Cu and Ni, as materials for internal conductors, i.e., internal electrodes, has also been implemented. Recently, a further decrease in the thickness of layers has been achieved, and dielectric ceramic layers with a thickness of approximately 1 xcexcm have been developed.
When dielectric materials for such monolithic ceramic capacitors are produced, various metallic elements are often added to the basic ceramic powder, such as a barium titanate powder, in order to modify electrical characteristics and to improve sinterability. Examples of known methods for adding such metallic elements to basic powders are as follows:
(1) A method in which carbonates or oxides of additive metallic elements, or the mixtures thereof, are added to a dielectric ceramic basic powder, mixing is performed, and then calcination is performed;
(2) A method in which an aqueous solution containing additive metallic elements is added to a slurry of a dielectric ceramic basic powder, precipitation is performed by adding a precipitant thereto, filtration and drying are performed, and then calcination is performed;
(3) A method in which a slurry is formed by adding a binder and a solution containing the compounds of additive metallic elements to a dielectric ceramic basic powder, green sheets are formed by a doctor blade process or the like, and then firing is performed (see Japanese Unexamined Patent Application Publication Nos. 5-89724 and 5-144319); and
(4) A method in which a dielectric ceramic basic powder, an organic solvent and a surfactant are mixed and pulverized to form a slurry, a complex alkoxide solution containing additive metallic elements is added to the slurry, particles of the ceramic basic powder having surfaces coated with the complex alkoxide containing the additive metallic elements are obtained by removing the organic solvent, and then calcination is performed (see Japanese Unexamined Patent Application Publication No. 10-139553).
Since powders are mixed with each other in method (1), it is not possible to homogeneously disperse the carbonates or oxides of the additive metallic elements in the basic powder on the microscopic level. Therefore, the additive effect varies with the individual particles of the basic powder since the additive metallic elements segregate in the ceramic basic powder, and the variation in the characteristics of the ceramic electronic components is increased, and also it becomes difficult to obtain desired characteristics.
In method (2) described above, the dispersibility is not sufficient, although it is improved compared to method (1), and also it is not possible to coprecipitate a plurality of metal ions by one type of precipitant. For example, although it is possible to precipitate Sr+2, Ca+2 and Mg+2 ions as complex carbonates by reaction with carbonate ions, it is not possible to precipitate Ti+4 ions and the like as carbonates using carbonate ions.
Should a metal-free ammonium salt be used as the precipitant, precipitates cannot be generated since Zn+2, Mn+2, Ni+2, Co+2 ions and the like form soluble amine complexes.
In method (3) described above, the organic binder is gelated depending on the types of anions or cations present during the formation of the slurry because of an interaction with the ions, and therefore it may become impossible to homogeneously disperse the additives.
In method (4), the gelation of the binder can be suppressed because after the surfaces of the particles of the basic powder are coated with the complex alkoxide containing the additive metallic elements, calcination is performed, and then a binder solution is added thereto to form a slurry. However, during the calcination, both the compound of the metallic element added to modify the electrical characteristics and the compound of the metallic element for forming glass added to improve the sinterability are decomposed. The component for modifying the electrical characteristics preferentially reacts with the glass component, and as a result, it may not be possible to obtain a satisfactory characteristic modification effect, or the softening temperature of the glass may be changed and thereby its function as the sintering aid is not sufficiently carried out. The same problem also arises in methods (1) to (3) described above.
It is an object of the present invention to provide a raw ceramic powder and a method for making the raw ceramic powder in which a metal compound for modifying electrical characteristics (hereinafter referred to as an xe2x80x9celectrical characteristic modifierxe2x80x9d) and a metal compound for improving sinterability (hereinafter referred to as a xe2x80x9csintering aidxe2x80x9d) are formed on the surfaces of the individual particles of a ceramic basic powder without impairing the functions as the electrical characteristic modifier and the sintering aid. It is another object of the present invention to provide a dielectric ceramic produced using the raw ceramic powder and a monolithic ceramic electronic component using the dielectric ceramic.
In one aspect of the present invention, a method for making a raw ceramic powder includes the steps of mixing and pulverizing a basic powder which is a principal ingredient of the raw ceramic powder, an organic solvent, a first organic metal compound which is soluble in the organic solvent and contains a metallic element for modifying the electrical characteristics of the basic powder, and a second organic metal compound which is soluble in the organic solvent and contains a metallic element for improving the sinterability of the basic powder to form a slurry; removing the organic solvent from the slurry to obtain the basic powder, the surfaces of the particles of the basic powder being coated with the first organic metal compound and the second organic metal compound; and then heat-treating the basic powder at a temperature higher than the decomposition temperature of the first organic metal compound but lower than the decomposition temperature of the second organic metal compound.
Preferably, a surfactant is added into the slurry. Also preferably, the first organic metal compound is at least one metal alkoxide, metal acetylacetonate ord metallic soap.
The second organic metal compound is preferably a complex metal alkoxide formed by the reaction of a silica sol, a metal acetate and a polyhydric alcohol.
In another aspect of the present invention, a raw ceramic powder includes a basic powder which is a principal ingredient of the raw ceramic powder, a first organic metal compound containing a metallic element for modifying the electrical characteristics of the basic powder, and a second organic metal compound containing a metallic element for improving the sinterability of the basic powder, the first organic metal compound and the second organic metal compound adhering to the surfaces of the particles of the basic powder, the basic powder being heat-treated at a temperature higher than the decomposition temperature of the first organic metal compound and lower than the decomposition temperature of the second organic metal compound.
Preferably, the basic powder is a BaxTiO2+x powder, wherein 1.00xe2x89xa6xxe2x89xa61.03. Preferably, the c-axis/a-axis ratio of the basic powder crystals is in the range of about 1.003 to 1.01. Also preferably, the basic powder has an average particle size of about 50 to 200 nm and a maximum particle size of about 300 nm or less.
In yet another aspect of the present invention, a dielectric ceramic is produced by firing the raw ceramic powder.
In another aspect of the present invention, a monolithic ceramic electronic component includes a laminate including a plurality of dielectric ceramic layers, and a plurality of internal electrodes extending along the predetermined interfaces between the dielectric ceramic layers, the internal electrodes being placed so as to form a capacitance between the two adjacent internal electrodes separated by one of the dielectric ceramic layers, the dielectric ceramic layers being composed of the dielectric ceramic described above.
Preferably, the internal electrodes contain a base metal as a principal ingredient. More preferably, the base metal is either nickel or a nickel alloy. Further, the dielectric ceramic layer placed between the two adjacent internal electrodes preferably, has a thickness of about 1 xcexcm or less.